It is more especially applied in the manufacture of transformers without magnetic cores for medical instruments used for the formation of optical images by magnetic resonance. It is known that, to make these devices function properly, it is desirable to eliminate factors that cause distortions in the field lines of the magnet which is incorporated into these devices. However, at the operating frequencies (6 to 30 MHz) of these instruments, the techniques of the prior art for building current transformers entail the use of ferrite magnetic circuits to obtain efficient magnetic coupling between the primary and secondary coils of the transformers. These manufacturing techniques naturally run counter to the design requirements of instruments for optical image formation by magnetic resonance. This is because the presence of a ferrite current transformer can naturally give rise to distortions in the field lines of the magnet in instruments for optical image formation by magnetic resonance.